1. Field of the Invention
The present invention relates to a crossed dual tag the communication distance of which with a reader/writer is extended by crossing two linear polarized wave tags at right angles, and to an RFID (Radio Frequency IDentification) system using the crossed dual tag.
2. Description of the Related Art
Conventionally, RFID systems are put into practical use as a non-contact authentication technology for recording various items of information about a person or an object in an IC (Integrated Circuit) chip of an RFID, and for wirelessly reading the information with a reader/writer.
The RFID is generally referred to as wireless tag or merely as tag although it is called in a variety of ways.
A tag is composed of a metal antenna formed on a plane of a sheet, a film, etc. with a thickness of approximately 0.1 mm, and an IC chip connected to the feeding point of the antenna.
Normally, an IC chip is extremely small, and its thickness and area size are on the order of 0.2 mm and a 1-mm-square respectively. An antenna connected to the IC chip is formed as a dipole antenna pattern, and its communication wave generated by a resonant current is a linear polarized wave.
In an RFID system, a reader/writer transmits a signal of a wireless radio wave of approximately 1 W, the side of a tag receives the signal and returns information within the IC chip as a response signal to the side of the reader/writer, and the reader/writer reads the information.
The tag does not include a battery. When the tag makes an approach to the reader/writer, an electric current is generated by the resonation of the antenna of the tag with the radio wave emitted by the reader/writer. Only at this moment, the circuit of the IC chip operates to transmit the information therein to the reader/writer.
The antenna on the side of the reader/writer is formed to emit a circular polarized radio wave for enabling a communication with the tag generating a linear polarized wave for its communication regardless of any direction of the orientation of the tag.
UHF (Ultra High Frequency) ranging from 860 to 960 MHz is applied as the radio wave used for this transmission. For example, UHF of 952 to 954 MHz is used in Japan.
A communication distance between the reader/writer and the tag is approximately 3 to 5 m under an ideal condition although it depends on the gain of the antenna of the tag, the operational voltage of the IC chip, an ambient environment, or the like.
The antenna of the reader/writer makes a communication by using a circular polarized radio wave, whereas the antenna of the tag makes a communication by using a linear polarized wave. Therefore, power produced by resonating with the radio wave received from the reader/writer, namely, the power received from the reader/writer is one half of that in the case where the tag is assumed to generate the circular polarized wave.
Transmission power is attenuated in inverse proportion to the square of a distance. Therefore, the above described actual communication distance 3 to 5 m is reduced to 1/√2=1/1.41 of that in the case where also the tag generates a circular polarized wave.
As the tag intended to merely emit a circular polarized wave, a configuration where two dipole antennas are crossed at right angles, two feeding terminals for connecting one of the dipoles of one dipole antenna to one of the dipoles of the other dipole antenna and for connecting the other dipole of one dipole antenna to the other dipole of the other dipole antenna are provided, and an IC tag and a circuit having a phase difference of π/2 are connected between the two feeding terminals is proposed (for example, see Patent Document 1).
When the tag is implemented as the tag generating a circular polarized wave as described above, the two orthogonal dipole antennas using linear polarized waves must be connected with the circuit having the phase difference of n2, namely, a 90° phase shifter.
The tag normally has a simple structure implemented by directly connecting a dipole antenna pattern to a small IC with an area size of an approximately 1-mm-square. Therefore, the structure that requires a 90° phase shifter in addition to the IC as disclosed by Patent Document 1 leads to an undesirable increase in an overall cost.
Additionally, the technique disclosed by Patent Document 1 is the tag configured specifically for generating a circular polarized wave, in which the dipole antennas are crossed at right angles and prevented from being separated, and connected to the IC tag and the 90° phase shifter. This tag must be specifically designed and manufactured with dedicated process steps, leading to lack in the degree of arbitrariness of design.    [Patent Document 1] Japanese Published Unexamined Application No. 2003-249820